Boarding ladders or stairways are typically needed on ships, large land vehicles and mobile machines. One type of installation that poses requirements which are satisfied by the retractable boarding ladder of the present invention is a large crawler-mounted digging machine such as is used in strip mining operations. Such a machine has a revolving platform that is mounted over its crawler treads and carries the upper works of the machine including its digging tool. An operator's cab is mounted at a substantial elevation above the platform. From the level of the platform, access to the cab can be by way of stairways or the like that are fixed on the upper works; but the platform is about 10 or 12 feet above ground level, and access to it should be by way of a ladder or stairway that can be retracted so as not to interfere with swinging of the upper works or with movement of the machine on its crawler treads. As will be evident, the most desirable retracted position for this ladder or stairway is a raised one in which its bottom is at or above the level of the platform and in which no portion of it projects laterally beyond the boundaries of the platform.
There are certain other features which are highly desirable in such a boarding ladder or stairway, but these have heretofore posed problems that have not been easy to solve. As a matter of preference, the boarding device should take the form of a stairway rather than a ladder. Ascent or descent of a ladder requires a person to use one hand to hold onto the ladder in order to maintain his balance, so that it is difficult for him to carry any substantial load of tools or the like, whereas it is relatively easy to carry a heavy load along a stairway that extends at a substantially oblique angle to the vertical, has treads of substantial width, and has suitable guard rails.
In the past, however, where retractability has been essential, a ladder as such has usually been used, rather than a stairway, because it has been accepted that satisfactory retraction and extension could not be achieved with a stairway. A stairway is inherently much heavier than a ladder, and its weight thus poses problems with respect to raising and lowering it. Although power means could be employed for moving it up and down, a power failure would then leave the stairway inaccessible at a time when its availability would be very important.
U.S. Pat. No. 273,386 discloses the use of a manually operated winch for raising and lowering a heavy retractable ladder or stairway, but that expedient is obviously slow and cumbersome.
U.S. Pat. No. 3,737,007 discloses the employment of a counterweight or counterweights to offset the weight of a ladder so that it can be easily raised and lowered. However, the counterweight arrangement shown in that patent, or any other ordinary and obvious counterweight arrangement, would involve significant disadvantages. If the counterweight overbalances the ladder or stairway element, that element will tend to move up out of its extended operative position and to accelerate steadily as it continues to its retracted position, whereas if the retractable element overbalances the counterweight, the ladder or stairway will be hard to raise and will not tend to remain in its raised position but will instead accelerate all the way down to its operative position, where it will be brought to a jolting stop.
The retractable ladder of U.S. Pat. No. 3,737,007 was always raised and lowered from a location at its foot, and it was held in any selected raised position by locking means accessible from below. A boarding ladder for a mining machine or the like must be capable of being both raised and lowered from both above and below, and therefore latching it in either of its positions is not practical.
Of course a stairway, as distinguished from a ladder, presents the further complicating problem that it should undergo a change of orientation as it moves between its retracted and its operative positions. A ladder can move straight up and down between those positions, whereas a retractable stairway should extend at a substantially oblique angle to the vertical when in its lowered operative position, but when retracted it should be as nearly vertical as possible so that it has minimum lateral extension and maximum compactness. Obviously the stairway should not require manual guidance or control for the accomplishment of this change in its orientation.
In some prior boarding stairway installations for mining machines and the like, the stairway could be in its operative position only when the platform of the machine was in designated positions of its swinging motion. Such a limitation is obviously undesirable, as would be any limitation that might be imposed upon swinging of the upper works by the presence of a boarding ladder or stairway in its retracted position.
Before the present invention was made, a great amount of consideration was given to the use of springs for offsetting the weight of a retractable stairway to facilitate manual raising and lowering of it and to provide an arrangement that would satisfy all of the above discussed requirements. However, such proposals involved structural complications or required springs that were so large as to be very expensive and difficult to handle. The powerful springs needed with a stairway had the further important disadvantage that they posed a threat of personal injury in the event of their breakage.